Right we know that IP = EJ and that IJ = EP that's because there's only two ways of doing it and four groups to cover. However how does this rationalise out? What is it about introverted perceptives which is parallel to extroverted judgementals aside from their function order?

I think that this will be useful in understanding the system as a whole. After this point there will be the last piece of the puzzle fixed.

See if the person is NT then their preferences are either NT or TN.
If the person is SF then the preferences are either SF or FS.

The piece that is missing is which one is first and is it extroverted or introverted.

Basically there are 4 not 16 broad groups of people. NT, NF, ST & SF. This is where Jung and the MBTI start.

Within these groups there are four types. EJ, EP, IJ & IP. So we can quite reasonably say that it's not really 16 types per se but four groups and within each there are four temperaments.

However this only give us the MBTI system, it doesn't say why the system hangs together, which is what we are querying.

Right we know that IP = EJ and that IJ = EP that's because there's only two ways of doing it and four groups to cover. However how does this rationalise out? What is it about introverted perceptives which is parallel to extroverted judgementals aside from their function order?

I think that this will be useful in understanding the system as a whole. After this point there will be the last piece of the puzzle fixed.

See if the person is NT then their preferences are either NT or TN.
If the person is SF then the preferences are either SF or FS.

The piece that is missing is which one is first and is it extroverted or introverted.

Basically there are 4 not 16 broad groups of people. NT, NF, ST & SF. This is where Jung and the MBTI start.

Within these groups there are four types. EJ, EP, IJ & IP. So we can quite reasonably say that it's not really 16 types per se but four groups and within each there are four temperaments.

However this only give us the MBTI system, it doesn't say why the system hangs together, which is what we are querying.

So why are IPs similar to EJs?
Why are IJs similar to EPs?

IP is conditionally EJ. IJ is conditionally EP.

Judgemental versus non judgemental condition.

Point counter point. Each point counter each point. How many groups in 16?